The mass murder of Native children in Canada from 1832 - 1996, by the Church of England, the English State and the Vatican. The bodies of these 'disappeared' children were continually hidden in the extensive grounds of the 'school' they were incarcerated in. Their graves were then covered with tons of earth.

I am deliberately being non-pc in saying "man's" inhumanity. Almost always it is man - the male of the species - who behaves in such ways. Perhaps an echo of behaviour from the time when humankind had moved on only a little from the 'primitive' form.